This text comes from The Complete Poetical Works of John Greenleaf Whittier, a historical songbook transcribed on Wikisource and reproduced from the pinned revision linked below.
The threads our hands in blindness spin
No self-determined plan weaves in;
The shuttle of the unseen powers
Works out a pattern not as ours.
Ah! small the choice of him who sings
What sound shall leave the smitten strings;
Fate holds and guides the hand of art;
The singer’s is the servant’s part.
The wind-harp chooses not the tone
That through its trembling threads is blown;
The patient organ cannot guess
What hand its passive keys shall press.
Through wish, resolve, and act, our will
Is moved by undreamed forces still;
And no man measures in advance
His strength with untried circumstance.
As streams take hue from shade and sun,
As runs the life the song must run;
But, glad or sad, to His good end
God grant the varying notes may tend!